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16<title>GNU gprof: Assumptions</title>
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62<a name="Assumptions"></a>
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64<p>
65Previous: <a href="Sampling-Error.html#Sampling-Error" accesskey="p" rel="previous">Sampling Error</a>, Up: <a href="Inaccuracy.html#Inaccuracy" accesskey="u" rel="up">Inaccuracy</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>]</p>
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68<a name="Estimating-children-Times"></a>
69<h3 class="section">6.2 Estimating <code>children</code> Times</h3>
70
71<p>Some of the figures in the call graph are estimates&mdash;for example, the
72<code>children</code> time values and all the time figures in caller and
73subroutine lines.
74</p>
75<p>There is no direct information about these measurements in the profile
76data itself.  Instead, <code>gprof</code> estimates them by making an assumption
77about your program that might or might not be true.
78</p>
79<p>The assumption made is that the average time spent in each call to any
80function <code>foo</code> is not correlated with who called <code>foo</code>.  If
81<code>foo</code> used 5 seconds in all, and 2/5 of the calls to <code>foo</code> came
82from <code>a</code>, then <code>foo</code> contributes 2 seconds to <code>a</code>&rsquo;s
83<code>children</code> time, by assumption.
84</p>
85<p>This assumption is usually true enough, but for some programs it is far
86from true.  Suppose that <code>foo</code> returns very quickly when its argument
87is zero; suppose that <code>a</code> always passes zero as an argument, while
88other callers of <code>foo</code> pass other arguments.  In this program, all the
89time spent in <code>foo</code> is in the calls from callers other than <code>a</code>.
90But <code>gprof</code> has no way of knowing this; it will blindly and
91incorrectly charge 2 seconds of time in <code>foo</code> to the children of
92<code>a</code>.
93</p>
94<p>We hope some day to put more complete data into <samp>gmon.out</samp>, so that
95this assumption is no longer needed, if we can figure out how.  For the
96novice, the estimated figures are usually more useful than misleading.
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